The present invention relates to an ultrasonic sensor.
So-called diaphragm- or miniature hydrophones are used in the determination of the characteristics of an ultrasonic field prevailing within a sound-carrying medium such as water. The three-dimensional distribution of the sound-pressure amplitude of the ultrasonic field is detected in that the prevailing sound-pressure is measured with this type of hydrophone at different locations in a measuring tank. During this measurement, these hydrophones should influence the sonic field to be measured as little as possible.
For example, in "Ultrasonics, May 1980, pp. 123 to 126" a diaphragm hydrophone is revealed in which a foil of polyvinylidene fluoride PVDF with a thickness of 25 um is clamped between two metal rings serving as supporting bodies. In this manner, a diaphragm with an inside diameter of approximately 100 nm is formed. The surfaces of the diaphragm are provided, in a small central region, with disk-shaped electrodes lying opposite each other whose diameter amounts to 4 mm, for example. Between these electrodes there is the polarized piezoelectrically active region of the diaphragm. Supply leads mounted on the surface of the diaphragm as metallic films run from the disk-shaped electrodes to the rim of the diaphragm, and there, with the aid of conductive adhesive, they make contact with a coaxial cable.
In the case of this known hydrophone, however, problems can arise during the measurement of ultrasonic shockwaves, especially in the case of frequent application. These kinds of shockwaves having very steep pulse edges whose rise times are less than 1 usec and whose pressure amplitudes lie in the range of 10.sup.8 Pa, lead over time to a mechanical destruction of the metallic electrodes attached to the PVDF layer, which (mechanical destruction) is caused by cavitation effects. Such shockwaves appear e.g., in the focus range of lithotripters in which a focused ultrasonic shockwave is used to destroy concrements, e.g., kidney stones in the kidney of a patient. Not only during development, but also during the routine monitoring of these types of apparatus, it is necessary to determine the characteristics of the shockwave in the focus range.
An especially suitable ultrasonic sensor for measuring such shockwaves is known from European Patent Application A2-0 227 985, in which a polymeric foil, which is fastened at its rim area to a supporting body, is piezoelectrically activated in a sub-range and is coupled electrically to electrodes which are mounted in spatial separation from the piezoelectrically active area. The surface charge oscillations, created by an ultrasonic wave, within the piezoelectrically active range of the polymeric foil are coupled electrically via the sound-carrying medium which surrounds the polymeric foil to the electrodes mounted outside of the surface area of the polymeric foil assigned to the piezoelectrically active region of the polymeric foil. The piezoelectrically active central region of the polymeric foil can thus be designed within the focus range of a focused ultrasonic shockwave since there is no mechanically unstable, electrically conductive layer present in the central region of the polymeric foil.
A purely capacitive coupling without high signal loss is possible by means of the use of a piezoelectric polymer with a dielectric constant that is relatively low in comparison with piezoceramic materials. The electrodes can accordingly be mounted in spatial separation from the piezoelectrically active region of the polymeric foil as well as on the foil itself as well as outside of the foil, too, e.g., on the supporting body of the foil.
In the case of lithotripters which are mounted in a water tank in which the patient is also located for treatment, supervision of the focus by means of the ultrasonic sensors is possible without any problems These ultrasonic sensors must simply be placed in the water tank at hand for supervisory purposes. Newer lithotripters, however, are comprised of a flexible shockwave transmitter, which as a rule is coupled to the body of a patient via a saccular, rubber diaphragm encased, water-filled delay path (EP-A2-0 133 665). A water tank, in which the patient must take a seat and in which an ultrasonic sensor can be mounted for supervisory purposes, thus no longer exists.
For the practical implementation of routine measurements in the focus range of this kind of lithotripter, however, is desirable that these routine measurements be possible without more extensive preparation and with little effort. This is not possible with the known ultrasonic sensors. As a rule, the shockwave transmitter must be removed and be checked at a separate measuring location in a measuring tank prepared specifically for this purpose.